The present invention relates to an improvement to a gas/liquid ultrasonic flowmeter as described in detail in various embodiments in the older German patent applications P 39 41 546.5 (corresponding to PCT publication WO 91/09282) and P 39 41 545.7 (corresponding to U.S. Ser. No. 860,503) and P 39 41 544.9 (corresponding to U.S. Ser. No. 861,861). It is common to the flowmeters of the aforementioned older patent applications that the measurement tube through which the gas or the liquid flows has a rectangular cross-section in the region of the ultrasonic flow measurement. This means that this measurement tube possesses plane sidewalls. It is also common to all these flowmeters that the ultrasound employed for the flow measurement exhibits a W-shaped path with correspondingly three reflections of the ultrasound on such walls of the measurement tube. The expression "W"-shaped" refers to the principle of the path route of the ultrasound. It is also possible to provide ultrasound paths with even more successive reflections between the emission transducer generating the ultrasound and the reception transducer receiving the ultrasound. The fitting of the reception/transmission transducers which must necessarily be provided and the requirement to fit these transducers to the measurement tube so that no significant impairment of the flow of the gas or of the liquid in the measurement tube takes place require the injection of the ultrasound in an oblique direction into the measurement volume of the measurement tube and the ultrasound to be received again by the reception transducer in a corresponding oblique direction.
The aforementioned patent application P 39 41 546.5 relates to a flowmeter with such a rectangular cross-section, the height H of which to the width B of the cross-sectional surface of the flow is greater than 2:1 to 15:1. With such dimensioning of the side relation of the walls of the measurement tube, gas or the liquid flows homogeneously through this portion of the measurement tube, specifically if the transmission/reception transducers are fitted to the measurement tube on the narrow sides having the width B and the height dimension of the cross-section is a component of the path of the ultrasound in the measurement tube. Further pertinent details of the geometric arrangement are evident from the cross-sectional representation in FIG. 1 and the side elevation in FIG. 2.
FIGS. 1 and 2 show, taken together, the elevation in the axial direction of the flow (FIG. 1) and the side elevation of the tube cut in the longitudinal direction (FIG. 2). H and B indicate the above defined dimensions for the height and width of the rectangular tube 1. 2 and 3 designate the sidewalls, and 4 and 5 designate the lower sidewall and the upper sidewall. The same reference symbols are shown in FIG. 2. The transducers 11 and 12 are inserted into the upper sidewall 5, specifically with their transmission and reception surfaces 111 and 112, respectively, directed obliquely to the sidewall 5 (as shown in FIG. 2). The W-shaped ultrasound path 21 is directed at the angle alpha (=a), namely related to the sidewalls 4 and 5. The surfaces 111 and 112 are directed so that the ultrasound path 21 strikes these surfaces perpendicularly.
In principle, however, it cannot be avoided that a component of acoustic energy is also radiated at such an angle from the surface 111 or 112 or is received at an angle from these surfaces that leads to a V-shaped path 22 in the tube interior of the tube 1. The V-shaped path having the angle beta (=b) has a different transit time than the desired W-shaped path, which advantageously has a greater acoustic path length in the tube 1 than is the case for the V-shaped path.
In FIG. 2, the overall length of the flowmeter is indicated by L. The other illustrations in FIG. 2 relate to the physical explanation of the invention which is given in the following text.